1. Field of the Invention
Aspects of the present invention relate to an organic electrolytic solution and a lithium battery including the same, and more particularly, to an organic electrolytic solution in which decomposition of an electrolyte included in an organic electrolytic solution is prevented to improve cycle characteristics of a battery including the organic electrolytic solution, and in which an additive that can prevent cycle deterioration is introduced to improve performance of a battery including the organic electrolytic solution, as well as a lithium battery with improved cycle characteristics as a result of including the organic electrolytic solution.
2. Description of the Related Art
Due to the development of electronic industries, portable and wireless electronic devices, such as portable phones, digital cameras, notebook computers, and the like have been widely used. Thus, there is an increasing demand for secondary batteries that are lightweight, small in size, and have a high energy density, as a driving power source of such electronic devices. Of these secondary batteries, research has been actively conducted into lithium batteries that comprise a non-aqueous electrolytic solution, a lithium-containing metal oxide having a voltage of 4 V as a cathode active material, and a carbon material that can insert and remove lithium as an anode. Such batteries have a high voltage and high energy density.
The average discharge voltage of lithium batteries is in a range of about 3.6 to about 3.7 V, which is relatively higher than that of other alkali batteries, nickel metal hydride batteries, or nickel-cadmium batteries. However, in order to attain such a high operating voltage, it is necessary to form an electrolytic solution that is electrochemically stable at a charge/discharge voltage within a range of 0 to 4.2 V. To this end, a mixture of a cyclic carbonate-based solvent such as ethylene carbonate, propylene carbonate, butylene carbonate, or the like is widely used as the electrolytic solution.
In the case of an ethylene carbonate-based electrolyte, stability and thermal stability of a battery are decreased because of electrolyte decomposition at a high voltage and subsequent gas generation. To address these problems, research into propylene carbonate-based electrolytes having an excellent stability at a high voltage has been actively conducted. However, when a propylene carbonate-based electrolyte is used, solvent decomposition and co-insertion of lithium and electrolyte continuously occur on the surface of the negative electrode. Thus, it is difficult to form a uniform solid electrolyte interface (SEI) film. That is, the solvent and lithium ions are simultaneously inserted into carbon materials, and thus charge-discharge efficiencies are decreased.
After being formed at an initial stage, the SEI film prevents lithium ions from reacting with a carbon negative electrode or other materials during charging and discharging, thereby acting as an ion tunnel. The ion tunnel prevents disintegration of the structure of the carbon negative electrode from co-insertion of lithium ions and high molecular weight organic solvent molecules moving with the lithium ions by solvation at the carbon negative electrode. Thus, in the formation of the SEI film, lithium ions do not react again with the carbon negative electrode or other materials. As a result, the concentration changes of lithium ions are reversible and can be maintained within acceptable ranges.
However, as charge-discharge cycles occur, expansion and contraction of an electrode plate occur repeatedly and a partial over-voltage also occurs. Because of these changes, a passivation layer such as an SEI film slowly disintegrates as time goes by. Thus, a side reaction with the surface of the anode exposed to the surrounding electrolytic solution occurs continuously. Accordingly, gases such as carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ethane (C2H6), or the like are generated depending on the type of carbonate used and the anode active material. Because of this continuous generation of gases, the pressure in the battery increase and the battery's cycle characteristics decrease significantly.
In addition, a carbonate-based electrolytic solution is decomposed by a graphite-based anode active material, and delamination of carbon materials occurs. As a result, capacitance, cycle characteristics and retention characteristics of the battery decrease. In particular, such a phenomenon significantly occurs in an electrolytic solution containing propylene carbonate. In addition, during initial charging, the propylene carbonate decomposes on the anode, resulting in a large reduction of initial capacity.
As a method of preventing decomposition of cyclic carbonates by the graphite-based anode active material and delamination of carbon materials, adding a crown ether to an electrolytic solution based on propylene carbonate and ethylene carbonate has been proposed. However, this is not practical because a large amount of the crown ether, which is expensive, should be added, resulting in poor economic efficiency, and insufficient improvement in battery characteristics.
Japanese Patent Laid-Open Publication No. hei 8-45545 (corresponding to U.S. Pat. No. 5,626,981) discloses a method of adding vinylene carbonate to an electrolyte based on propylene carbonate and ethylene carbonate in order to prevent decomposition of the electrolyte. According to this method, the vinylene carbonate is reduced at a graphite anode during charge cycles and forms an insoluble film on the surface of the anode, thereby preventing reduction of solvents such as propylene carbonate and ethylene carbonate.
However, this method using vinylene carbonate alone cannot accomplish the formation of a complete SEI film at the first charge cycle. As charge and discharge cycles are repeated at room temperature, the film may crack and vinylene carbonate decomposes and is used up. Ultimately, it is not possible to obtain the desired stable cycle life characteristics of a battery. Further, although cycle life characteristics of a battery may improve by increasing the amount of vinylene carbonate, the method still has problems in that the discharge capacity of a battery decreases rapidly at low temperature and swelling of a battery may occur when the battery is stored at high temperature.